Electrostatic loud speaker



Feb. 27, 1934. H. VOGT 1,948,637

ELECTROSTATIC LOUD SPEAKER Filed Oct. 7, 1930 2 Sheets-Sheet l l 7 I g! 1 5955 I jg'y Jaflezatar:

v H. voe'r ELECTROSTATIC LOUD'SPEAKER Feb. 27, 1934.

F iled Oct. 7, 1930 2 Sheets-Sheet 2 Patented Feb. -27, 1934 1,948,637 a ELECTROSTATIC LOUD SPEAKER Hans Vogt, Berlin, Germany Application. October '7, 1930, Serial No. 487,036

In Germany June 26, 1930 2 Claims. (Cl. 179-111) If a body, such as for instance a ,properly mounted diaphragm, is setinto vibrations, the amplitude of vibration of the diaphragm will not be the same at all frequencies, during constant 5 excitation. In this case the vibrating body, such as for instance a diaphragm, will more readily respond to a certain definite frequency of vibration. This frequency, as known, is ordinarily termed the natural frequency" of the diae phragm and accordingly, thetone or sound produced thereby t e natural tone of the diaphragmor the vibration period of its own. It

is further known that a diaphragm does not only respond to an increased extent to the natural frequency but also theother frequencies which are a multipleof'the natural frequency. This phenomenon is of greatly detrimental effect for the production of sound, for instance, by means of electric loudspeakers which operate on the principle of a vibrating diaphragm and more especially in case electrostatic loudspeakers are used, the-tones positioned within the range of the natural frequency of the diaphragm being reproduced at essentially greater strength than all other tones. Apparently, this will result into non-uniformity of the reproduction of sound which will greatly impair the clearness and faithfulness of reproduction.

I The principal object of my present invention is to properly construct the diaphragm to prevent formation of too large amplitudes of vibration within the range of the natural frequency of the diaphragm. More particularly, my invention is designed to be used in connection with sound reproducing devices in which the sound is generated by means of a vibrating diaphragm, especially in connection with loudspeakers of the electrostatic type having afdiaphragm mounted between fixed perforated electrodes which are actuated by means of alternating electric charges which are similar to the vibrations of the sound to be reproduced.

In carrying out my invention the perforations or apertures which are provided in said fixed perforated electrodes or plates of an electrostatic loudspeaker are properly dimensioned to permit passage of the air which is caused to vibrate with the diaphragm through the perforations or apertures in the fixed electrodes, the velocity of the air passing therethrough being of such a value that the pressure or resistance which is offered to the vibrations of the diaphragm may be regulated to exert an increased damping action upon the diaphragm within the range of its natural frequency of vibration or the vibration period of its own. A regulation of this kindwill have the effect of attaining a more uniform and faithful reproduction of the acoustic performance by means of an electroe static loudspeaker.

My invention will be more fully understood by reference to the accompanying drawings which form part of this specification. The drawings represent in part an example of a new electrostatic loudspeaker constructed according 5 to my present invention and in part diagrams which serve to explain the principle of operation of my new loudspeaker.

In the drawings Fig. 1 represents a plain form of .vibration of a diaphragm, Fig. 2 a form of vibration of a diaphragm having two nodal lines positioned concentrically within the surface of the diaphragm and Fig. 3 the sound pressures exerted upon the diaphragm of my new electrostatic loudspeaker, said sound pressures being shown as a function of the frequency of vibration by which the diaphragm is excited, Fig. 4 is a diametrical section through one form of electrode of a loudspeaker constructed according to my present invention, Fig. 5 a front view of so an electrode shown in Fig. 4 and Fig. 6 another diametrical section through a modified electrode of my new loudspeaker.

The vibrations of the diaphragm, when responding to the first or the second vibration period of its own or natural frequency, will follow the forms of vibration shown in Figs. 1 and 2, respectively. More particularly the full-drawn line in Fig. 1 shows the form which is assumed by the diaphragm in one of its outermost positions of vibration when set into vibration by a frequency within the range of the first natural frequency of the diaphragm, while the dotted line in Fig.

1 shows the diaphragm in its opposite outermost positionof vibration. In a similar way, Fig. 2 shows in a full-drawn and dotted line the outermost positions of vibration of the diaphragm set into vibration by a frequency within the range of its second vibration period of its own or natural frequency. In Fig. 3,.the dotted curve 2 repm0 resents the sound pressure generated by the ,vibrating diaphragm, said sound pressures being shown as a function of the frequency by which the diaphragm is set into vibration. In Fig. 3, furthermore the sound pressures are laid out-on the yertical axis and the frequencies on the horizontal. axis, the latter in a logarithmic scale. From the diagram of Fig. 3 it may be seen, for instance, that at a frequency of 80 Hertz the amplitude will be essentially greater than at .lfreno quencies above and below Hertz. The curve 2 of Fig. 3 furthermore shows that there is a second, somewhat greater maximum at the triple frequency, that is at a frequency of 240 Hertz.

The electrostatic loudspeaker as shown in Fig. 4 in a diametrical section consists in the known manner of the fixed perforated plates or electrodes 3, 3, the diaphragm 4 being stretched out between said electrodes in a manner to permit vibration of the diaphragm. The vibration of the diaphragm 4 is due to alternating electric fields which are generated intermediate the diaphragm and the fixed plates 3 and 3' by the aid of a suitable device, for instance a radio receiving set or the electrical tone receiver of a talking machine together with proper amplifying means. These auxiliaries are not shown in the drawings as they do not form part of my present invention.

According to my invention the vibrations of the diaphragm which are due to the natural frequency thereof are suppressed by air which acts as a damping medium. This damping action of the air is properly controlled according to my invention by giving a specific size to the perforations or apertures 5 in the plates or electrodes 3 and 3' permitting the passage of air therethrough according to 'my invention at the points of greatest amplitude of the vibration of the diaphragm as shown in Fig. 2, that is at the center of the diaphragm as well as at a distance from the center amounting to about two thirds of the radius of the diaphragm, measured from the center thereof. By giving the apertures or perforations 5 a proper size as above mentioned, a greater resistance will be offered to the air at these places of the diaphragm, the air being compressed within the interstices between the diaphragm and the electrode plates 3 and 3' in proximity thereto.

Thus, according to my invention the radial distances between the perforations which are arranged in concentrical circles are greater at a distance from the center, said distance amounting to about two thirds of the radius of the electrodes, than at the other places, for instance at the outermost periphery, where the width of the radial ribs is extremely small as compared with the width of the ribs '7, '7 and 7 as well as 8, 8' and 8". 1 I

In this manner zones of a higher air-pressure will be generated at the places of greatest deflection of thediaphragm during vibration there= of, said zones of higher air-pressure thus acting against undesirable excessive deflections of the diaphragm within the range of its vibration of its own or natural frequency, without, however,

permitting the damping action of the air to become unduly large to act unfavorably upon the deflections of normal size and to decrease thereby the strength of the sound reproduced by the apparatus in general. A proper relative decrease of the strength of the reproduced sound will thus be effected according to my invention as represented by the full-drawn curve 3 in Fig. 3.

In the modified form of perforated plate or electrode shown in Fig. 6 an increased resistance is offered to the passage of air through the apertures or perforations 5, at the same time retaining a uniform width of said apertures or perforations as wellas of the ribs 6, by providing a circular disc 9 at the middle part of the perforated plate or electrode and an annular disc at the portions of said plate or electrode which are positioned within a zone nearer the periphery, and by making the distance of the said two discs and the periphery of the plate regulable for the purpose of producing at these places a greater resistance against the passage of air through the plate.

By properly dimensioning the resistance offered to the air when passing out of the perforations or apertures 5 vin the fixed plates or electrodes and the specific effect of air-damping attained thereby the reproduction of sound by the diaphragm will be rendered essentially more unifornl within the entire range of frequencies, thus greatly improving the acoustic performance of the apparatus I claim:

1. An electrostatic loudspeaker, comprising a diaphragm mounted to permit vibration thereof, 110 a plate arranged on either side of said diaphragm, and segmentally shaped perforations provided in said plates, the segmental perforations at the middle part and at a zone which is at a distance of about two thirds of the radius from the center 115 of said plates being arranged at a greater distance from each other than within the remaining zones of said plates.

2. An electrostatic loudspeaker, comprising a diaphragm mounted to permit vibrations there- [20 of, a fixed plate arranged on either side of said diaphragm, said plates having segmentally shaped perforations, the distance between said segmentally shaped perforations varying from a greatest value near the center and in an annular 125 zone having a distance from the center amounting to about two thirds of the radius of the plate to thenormal distance-between said segmentally shaped perforations towards the periphery and the center. 

